Microwave ovens: My brother went on a catering course. One of the earliest parts of this involved the teacher putting a beaten-up ol' microwave oven in front of the class, and saying, "We'll show you this, so you don't try it for yourselves."

First event: putting a pin into the oven, which went ping-ping-ping as the microwaves kicked it around the inside.

Second event: a raw egg. Click, pause for half a second, then the inside of the oven was filled with shrapnel-filled meringue.

Third event, to drive the lesson home: "Now clean this out."